Canon

Whatever form it's told in (book, TV show, movie), a story will sometimes attract a body of pathologicalfans who want to make it the center of their own identities and social lives. Given the fans have:

reached a critical mass such that they are able to find each other

enough time on their hands and

an inadequate grasp of how stupid their obsessions sound,

they will spawn endless tl;dr words.
The new Internet-created environment for Condition 1 has broadened the scope of stories that fit this pattern.
If the story leads to subsequent spin-off stories (e.g. the story being told in a different (if perverted) form, and so on), the fans can enjoy endless debates about how the authentic original story was truly and properly told IMH fuckin' O and what should be allowed in those terms.

People in fandom also have flame wars concerning the terms "canonical" and "non-canonical". They always base their arguments on which fanfic holds their loyalty. This is because the people involved are always fucking retards.

Aside from the original work (even if it's a spin-off on a bubblegum wrapper), some fans will consider certain interviews with the Creator to be legitimately canonical. The fantards most guilty of this crime are the rabid, disillusioned shippers.

Take, for example, Harry/Hermione. Seriously, did you even read the fucking books? It's clear from the start that she was going to end up fucking Ron.

Any event in an episode of The Simpsons or part of a Simpsons movie is part of the Simpsons canon (despite lack of internal consistency); events in Simpsons comics are completely non-canonical, though.

In Lord of the Rings, Frodo and Sam being gay for each other is canon to the movie fans, but their Vegas marriage is only canon to the slashfic crew. Flame wars between the few who'd actually read the books and movie fans were also particularly hawt.